1. Field of the Invention
The present invention generally relates to signal generation, and more specifically to quadrature signal generation using an active polyphase inverter filter.
2. Background Art
In electronic communications, it is often useful to send and receive information using two or more signals that have a quadrature relationship. For instance, one information signal is designated the in-phase signal (I), and the other information signal is designated the quadrature signal (Q), where the Q signal is 90 degrees out of phase with the I signal. More specifically, the Q signal is delayed (or advanced) relative to the I signal (in-time) by 90 degrees.
Quadrature amplitude modulation (QAM) and quadrature phase shift keying (QPSK) are two well known specific examples of quadrature modulation.
The advantage of the quadrature signal transmission is that the bandwidth of a transmission medium is effectively doubled. In other words, if a particular transmission medium has a bandwidth of B (Hz), then quadrature modulation permits 2B (Hz) of information to be effectively transmitted through the medium without signal interference. This occurs because the I and Q signals occupy the transmission medium simultaneously, but are phase shifted with respect to each. At the receiver, the I and Q information can be discerned from each other by sampling the I and Q signals at the proper time based on the known 90 degree phase delay. However, the I and Q sampling times must be properly timed. Any error in sampling time will cause signal distortion and/or interference between the I and Q channels.
What is needed is a circuit and method to generate control signals that have precise timing to control sampling of I and Q signals. Furthermore, the circuit that generates the control signals should be able to be integrated on an integrated circuit.